In the design of vehicle airbag inflators, it is desirable to controllably generate a gas having selected properties, and for which the flow characteristics of the gas can be controlled during the gas generation. The specific requirements for the airbag inflator depend upon the airbag design and size, whether the airbag is for driver's side, passenger side or side impact application, the specific vehicle in which the airbag system is to be installed, its corresponding crash dynamics, and other factors. In each instance, there are exacting requirements for total gas volume generation, gas generation rate and time, gas toxicity, gas temperature and maximum operating temperature, gas particulate composition, storage requirements, and lifetime requirements.
The vehicle airbag inflator must be stable and safe in its pre-stored, pre-deployed state. This pre-deployed state may be quite prolonged, typically lasting one to several years, and often up to 20 years. During this pre-deployed period, the inflators routinely must endure a wide range of ambient temperatures, humidities, vibrational modes, and other harsh conditions. When called upon, the inflator must perform to specifications with high reliably.
In most applications, the airbag inflators also must be economical to produce and maintain. The airbag inflator should be as light and small as practicable, not only to lessen the direct cost of the inflator itself, but also to lessen the weight and size of the airbag unit for most efficient packaging and minimum mounting costs. This size and weight constraint can impact the inflator design, e.g., by dictating lower storage and operating pressures which allow for thinner pressure vessel walls.
Two design approaches have been widely used for vehicle airbag inflators. One design approach, commonly referred to as the pyrotechnic design approach, involves use of a pyrotechnic charge or propellant grain mounted in a pressure vessel. Upon deployment, an ignition device ignites the propellant, which causes it to react to produce hot gases. The pressure quickly builds in the pressure vessel until a pressure-sensitive output closure ruptures. This opening of the output closure allows the gases to be exhausted from the pressure vessel and out an exit port. The gases continue to flow in this manner until the propellant grain is completely consumed. Examples of such pyrotechnic designs include U.S. Pat. No. 3,985,076, entitled "Gas Generator," U.S. Pat. No. 4,907,819, entitled "Lightweight Non-Welded Gas Generator With Rolled Spun Lip," and U.S. Pat. No. 5,054,811, entitled "Arrangement for an Airbag Gas Generator, " in addition to others.
Pyrotechnic designs have been disadvantageous in vehicle airbag inflator applications in that they require careful control of the reaction rates so that flow rates of the system can be confined to within acceptable limits. The necessary features to provide this control add to the complexity, cost and risk associated with the design. The most important features that must be controlled in a pyrotechnic system are the burn surface area and the corresponding burn rate of the propellant, and the flow characteristics of the filter. The chemical nature of these devices also makes these pyrotechnic designs particularly susceptible to variations in ambient conditions such as temperature. The gas generation rate is undesirably high in warm ambient conditions, and it is undesirably low in cold ambient conditions. For example, the gas output rate for pyrotechnic designs can vary by as much as three to one over the typical operating temperature range for a vehicle airbag inflator system. Considering the various factors, and for the typical pyrotechnic inflator design, there can be significant variability in the gas output from lot to lot for commercial inflator units.
Pyrotechnic inflators designs also have been disadvantageous in that the gases produced in the devices often exceed permissible toxicity limits unless complex filtering is used. The use of filters complicate flow characteristics and add to unit weight and cost. This also has the corresponding disadvantage of making the expended inflators toxic, and requires in some instances that they be handled as hazardous wastes.
The second design approach is commonly referred to as the hybrid design approach. Hybrid designs involve the combination of a pyrotechnic inflator (use of a pyrotechnic charge to heat and/or generate gases as described above) and a pre-stored, pressurized gas. More specifically, a pressure vessel is used to pre-store a pressurized gas, typically an inert gas such as argon. A propellant, which may be contained within the pressure vessel or in a separate compartment, is disposed so that when ignited, the combustion products generated by the propellant come into intimate contact with the pre-stored gases. This quickly heats and expands the pre-stored gases, which builds the pressure in the pressure vessel until a pressure-sensitive output closure as described above ruptures to release the gas and exhaust it from the inflator. Examples of such hybrid designs include U.S. Pat. No. 5,060,974, entitled "Gas Inflator Apparatus," U.S. Pat. No. 5,257,819 entitled "Hybrid Inflator," and U.S. Pat. No. 5,290,060, entitled "Hybrid Gas Generator for Air Bag Inflatable Restraint Systems."
Hybrid inflators are subject to many of the same types of limitations as pyrotechnic inflators. In addition, hybrid designs have been unattractive for some applications in that they require storage of pressurized gas. This results in greater safety risks, thicker pressure vessel walls, larger size, heavier device weights, greater reliability concerns, and in some instances greater costs.